A material for substrate for an optical data recording medium has largely been glass or plastics, with a glass substrate having been used for an optical data recording medium such as optical disc memories for a computer which require high reliability, high recording density and long-term preservation. Glass is used because it has advantages in that: (1) it changes less with age in size and configuration caused by moisture absorption and the like; (2) it has less residual strain and thus double refraction does not arise; (3) it has excellent surface flatness and is difficult to be scratched; (4) it has high rigidity and thus is difficult to deformed; (5) it has excellent chemical and thermal durability; and (6) it can be accurately processed, and the like.
As disclosed, for example, in Japanese Patent Application Kokai (Laid-Open) No. 61-144740, a glass plate made by a so-called float method and subject to a chemically reinforcing treatment effected by such as an ion exchange method has been conventionally proposed as a glass plate for a glass substrate.
The glass plate made by the float method is characterized in that it is less expensive and has an excellent uniformity of thickness and flatness. Thus it is suitable as a glass substrate for optical data recording medium. In addition, the drawback of the glass plate in that it is fragile and liable to be broken is improved by the chemically reinforcing treatment.
By the way, since the float method is a method of making glass plates in such a manner that melted glass is flown onto the surface of melted metallic tin stored in a bath and moved in one direction, and semi-melted glass is stretched in the horizontal direction to enable the same to remove from the surface of the melted metallic tin and cooled, a mixed layer of metallic tin and a tin compound being formed on the surface (bottom surface) of the glass plate in contact with the surface of the melted metallic tin. On the other hand, the surface (top surface) of the glass plate which was not in contact with the melted metallic tin while it was made usually has not such a mixed layer formed thereon, but sometimes the mixed layer may be partially formed thereon because a vapor of the tin compound diffused in air is brought into contact with the glass plate. In any case, the mixed layer of the metallic tin and tin compound of high concentration is formed on the bottom surface of the glass plate rather than on the top surface thereof.
When the glass plate whose top surface and bottom surface have a different amount of the metallic tin and tin compound, as described above, is subject to the chemically reinforcing treatment by the ion exchange method in such a manner that it is dipped, for example, into a solution of potassium nitrate melted by being heated to a high temperature, the glass plate is warped. When, however, a glass plate not containing tin is dipped into the potassium nitrate solution as described above, it is not warped. This phenomenon of warping is considered to be caused by the difference of a doped amount of substituted ions on the upper and lower surfaces of the glass plate containing tin. More specifically, the chemically reinforcing treatment of the glass plate by the ion exchange method reinforces the glass plate in such a manner that alkali ions (for example, sodium ions) contained in the glass plate are substituted by other alkali ions (for example, potassium ions) having a diameter larger than that of the alkali ions contained in the glass plate to cause compression stress to remain on the surface of the glass plate. Thus, when the mixed layer of the metallic tin and tin compound is formed on the surface of the glass plate, tin atoms prevent the invasion of the substitution ions to result that a doped amount of a substituted ions (for example, potassium ion) in the surface of the glass plate where a larger amount of the tin is contained is made smaller than a doped amount of a substituted ions (for example, potassium ion) in the surface of the glass plate where a smaller amount of the tin is contained, whereby the above disadvantage is caused.
Since the above known example and prior art did not consider the problem in that the chemically reinforcing treatment is not uniformly effected due to the existence of the mixed layer at all, it has been difficult to make a glass substrate having a small amount of warping, and thus it has been difficult to make an optical data recording medium excellent in durability.